To remove unwanted deposits from various surfaces as noted above, a fluid stream should be employed which has a pressure in the range of about 5,000-20,000 psi, and typically about 10,000 psi. Such high pressure fluid streams can be controlled by a spray gun device which includes a body having a high pressure outlet to discharge the high pressure stream, and a low pressure "dump" outlet that relieves much of the pressure of the outlet stream when blasting is to be discontinued. A portion of the fluid flow through the body is directed to the dump outlet by a valve mechanism which is operably responsive to a hand-operated trigger. An example of the above dump-valve blast gun system is disclosed and as claimed in U.S. Pat. No. 4,602,740 issued Jul. 29, 1986, which is incorporated herein by reference.
Although blast guns built in accordance with the above-mentioned patent have numerous advantages and desirable features, in some applications is it desirable for economic reasons to minimize the volume of cleaning fluid that is used by eliminating the low pressure dump port. In that event the high pressure fluid stream must be controlled by a valve assembly which totally halts or shuts off the flow of high pressure fluid toward the outlet when the stream is to be stopped. Such a valve should also include an actuating lever, in the form of a trigger, by which the operator holds the valve closure member open unless and until such trigger is released, at which time the closure member should seat to shut off the flow.
The above-referenced U.S. Pat. No. 4,602,740 represents a different design approach in that a bypass valve mechanism, which prohibits during flow therethrough so long as a trigger handle is operatively engaged during blasting, is dimensioned such that a hydraulic bias force tends to shift the valve element toward its open or dump position. Thus when the trigger is released by the operator for any reason, the valve automatically opens to allow blast fluid to bypass therethrough so that the pressure and flow through the outlet of the gun are reduced. However in a shut-off type gun to which the present invention relates, a hydraulic or other bias force on the closure element in the opening direction is highly undesirable because the valve may not close when the actuator lever or trigger is released by the operator.
The opening and closing of the valve element to either start or stop the high pressure flow of the blasting medium results in sliding friction and wear between the closure element and its housing. Many closure elements are housed in a body that is a large, expensive component which retains high pressures and which, as the closure element is used, undergoes a considerable amount of wear also. Although the valve closure element typically can be replaced, the body can become worn to the extent that seal rings blow out under pressure and/or higher trigger forces are needed to operate the gun. Eventually the valve body must be replaced to restore the gun to a workable, like-new conditions, which requires a very expensive overhaul thereof.
In addition to the wear on parts due to sliding movement, damage to the valve seat surface can occur when foreign particles such as sand and rust in the fluid stream erode the seat surface as a high pressure fluid stream moves therepast at high velocity. Where such seat surface is formed on the valve body, time consuming and expensive overhaul is necessary to renew the gun assembly. Thus it is highly desirable to provide a valve closure element, and a housing or guide therefor which provides the seat in the form of a cartridge assembly that can be easily removed, repaired and replaced in the field, there being little chance of any wear problems with respect to the body itself since the critical wear surfaces are on the elements of the cartridge.
An object of the present invention is to provide a new and improved shut-off type high pressure blast gun where a hydraulic bias force is developed which tends to close the valve element and shut-off flow in the absence of an opening force thereon.
Another object of the present invention is to provide a new and improved shut-off blast gun of the type described which requires low actuating force in operation to reduce operator fatigue.
Still another object of the present invention is to provide a shut-off blast gun of the type described where the valve closure and its housing from a cartridge that is replaceable as a unit so that repair is easier, faster and less expensive than prior devices and can be readily accomplished in the field.